Abstract: The federal sentencing guidelines were created to reduce unwarranted sentencing disparities among similar defendants. This paper explores the impact of increased judicial discretion on racial disparities in sentencing after the guidelines were struck down in United States v. Booker (543 U.S. 220 [2005]). Using data on the universe of federal defendants, I find that black defendants received 2 months more in prison compared with their white counterparts after Booker, a 4 percent increase in average sentence length. To identify the sources of racial disparities, I construct a data set linking judges to defendants. Exploiting the random assignment of cases to judges, I find that racial disparities after Booker were greater among judges appointed after Booker, which suggests acculturation to the guidelines by judges with experience sentencing under a mandatory-guidelines regime. Prosecutors also responded to increased judicial discretion after Booker by charging black defendants with binding mandatory minimum sentences.